1. Field of the Invention
The present invention relates in general to a pump device having a plurality of pumps having different operating characteristics, and a hydraulic pressure control system including such a pump device, and more particularly to techniques for dealing with an operating failure of the pumps.
2. Discussion of the Related Art
A pump device of the type described above is disclosed in JP-A-9-256960. This pump device includes not only a plurality of pumps as indicated above, but also a pump control device for controlling those pumps, and a high-pressure pump and a low-pressure pump. The high-pressure pump has a highest delivery pressure which is higher than that of the low-pressure pump, and a highest delivery flow rate which is lower than that of the low-pressure pump. The pump device is provided in a braking system for an automotive vehicle, for delivering the pressurized fluid to wheel brake cylinders. The high-pressure and low-pressure pumps are selectively activated depending upon the desired fluid pressure to be applied to the wheel brake cylinders. When the desired pressure of a given one of the wheel brake cylinders is lower than a predetermined first threshold value, the low-pressure and high-pressure pumps are both activated. When the desired wheel brake cylinder pressure is between the predetermined first threshold value and a predetermined second threshold value larger than the first threshold value, only the low-pressure pump is activated. When the desired wheel brake cylinder pressure is higher than the second threshold value, only the high-pressure pump is activated. This selection of the pumps is based on a fact that a relatively large amount of the pressurized fluid should be the wheel brake cylinder for an initial period of a braking action of a hydraulic brake including the wheel brake cylinder, that is, until the hydraulic brake begins to provide a braking effect with respect to the corresponding vehicle wheel, while a relatively small amount of supply of the pressurized fluid to the wheel brake cylinder is required after the hydraulic brake has begun to provide the braking effect, that is, after the fluid pressure in the wheel brake cylinder has been raised to a relatively high level. Thus, the high-pressure pump and the low-pressure pumps are selectively used depending upon the required amount of supply of the fluid to be supplied to the wheel brake cylinder.
In the event of an operating failure of either one of those two pumps, the wheel brake cylinder pressure cannot be controlled to the desired value, or the amount of supply of the pressurized fluid to the wheel brake cylinder is insufficient, leading to deteriorated response in the control of the wheel brake cylinder pressure.
It is therefore a first object of the present invention to provide a pump device which includes a plurality of pumps having different operating characteristics and which is less likely to suffer from problems caused by an operating failure of some of the pumps.
A second object of this invention is to provide a hydraulic pressure control system including such a pump device.
The first or second object of this invention may be achieved according to any one of the following modes of the invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, so as to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only.
(1) A pump device (32) including a plurality of pumps having respective different operating characteristics, and a pump control device for controlling the plurality of pumps, characterized in that the pump control device comprises a control changing portion operable in response to detection of a failure of at least one of the plurality of pumps, for changing a manner of controlling at least one of the other of the plurality of pumps such that at least a portion of the operating characteristic of each of the above-indicated at least one of the plurality of pumps is compensated for by the changed manner of controlling the above-indicated at least one of the other of the plurality of pumps.
The operating characteristic of each pump includes a delivery pressure, a delivery flow rate, a rate of change of the delivery pressure and a rate of change of the delivery flow rate. In the pump device according to the above mode, the operating characteristic of each of the two or more pumps which is defective or fails to normally function will not be entirely lost, since the manner of controlling at least one of the other of the pumps is changed from the nominal manner, so as to compensate for at least a portion of the operating characteristic of the defective pump. Accordingly, a problem caused by the failure of one of the pumps can be minimized.
(2) A pump device according to the above mode (1), wherein the pump control device further comprises a pump failure detecting device for detecting the failure of the above-indicated at least one of the plurality of pumps, and the control changing portion operates in response to detection of the failure by the pump failure detecting device.
For instance, the pump failure detecting device includes a pump pressure sensor for detecting a delivery pressure of the pump device, and is adapted to detect a failure of at least one of the pumps on the basis of an output of the pump pressure sensor. The pump failure detecting device may be adapted to check whether the delivery pressure of each pump is zero or lower than a predetermined value during an operation of the pump or whether the delivery pressure has not been increased to a predetermined value a predetermined time after the initiation of an operation of the pump. In this case, the pump failure detecting device determines that the pump fails to normally function, if the delivery pressure is zero or lower than the predetermined value during an operation of the pump, or if the delivery pressure has not been increased to the predetermined value after the pump has been operated for the predetermined time. Although a user of the pump device may manually activate the control changing portion when the user has detected a failure of a certain pump, the failure is automatically detected by the pump failure detecting device, and the control changing portion is automatically activated in response to the detection of the failure, in the pump device according to the above mode. This arrangement assures relatively high accuracy of detection of the pump failure.
(3) A pump device according to the above mode (1) or (2), wherein the pump control device further comprises a pump selecting portion operable when the plurality of pumps are normal, for selecting at least one of the plurality of pumps which should be operated, on the basis of at least one of a desired delivery pressure and a desired delivery flow rate of the pump device, and on the basis of information defining an operating range in which each of the plurality of pumps should be operated, which operating range is determined by a capacity of each pump, and wherein the control changing portion changes the operating range of each of the above-indicated at least one of the other of the plurality of pumps.
Although the plurality of pumps have mutually different characteristics, the operating ranges of the pumps usually partially overlap each other, so that the pump selecting portion is desirably adapted to select the pump or pumps to be operated, such that each selected pump is operable under a load more or less smaller than its nominal or maximum load value. Therefore, upon failure of at least one of the pumps, the operating range of at least one of the other of the pumps is changed by the control changing portion so that the at least one other normal pump is selected and operated, in place of the at least one defective pump, such that at least a portion of the operating characteristic of each defective pump is compensated for by the operated normal pump or pumps. Accordingly, the problem caused by the failure of one or more pumps can be minimized.
(4) A pump device according to the above mode (3), wherein the plurality of pumps include a low-pressure pump and a high-pressure pump having a maximum delivery pressure higher than that of the low-pressure pump, and the operating range of each of the low-pressure and high-pressure pumps is defined by a desired value of the delivery pressure of the pump device, and wherein the control changing portion comprises a low-pressure-pump upper limit increasing portion for increasing an upper limit of a delivery pressure of the low-pressure pump to thereby change the operating range of the low-pressure pump when the high-pressure pump becomes defective, such that the upper limit is higher while the high-pressure pump is defective than while the high-pressure pump is not defective.
The low-pressure pump whose maximum delivery pressure is lower than that of the high-pressure pump is operated when the desired value of the delivery pressure of the pump device is relatively small, while the high-pressure pump is operated when the desired delivery pressure value of the pump device is relatively large. When the low-pressure pump and the high-pressure pump are both normally functioning, the upper limit of the range of the delivery pressure in which the low-pressure pump is permitted to be operated is lower than the maximum delivery pressure of the low-pressure pump. When the high-pressure pump is detected to be defective, the upper limit of the delivery pressure range in which the low-pressure is permitted to be operated is increased, so that the delivery pressure of the low-pressure pump can be made higher when the low-pressure pump is operated while the high-pressure pump is defective, than when the low-pressure pump is operated while the high-pressure pump is not defective. Accordingly, the delivery pressure obtained by the normally functioning high-pressure pump may be obtained by operation of the low-pressure pump when the high-pressure pump is defective.
(5) A pump device according to the above mode (4), wherein the pump control device commands the low-pressure pump to be held operated even in the absence of a command to increase a delivery pressure of the pump device while the high-pressure pump is normal, the pump control device further comprising a low-pressure turning-off portion operable when the high-pressure pump becomes defective, for turning off the low-pressure pump in the absence of the command, at least when the desired value of the delivery pressure of the pump device is in a range between two values of the upper limit of the delivery pressure of the low-pressure pump before and after the upper limit has been increased by the low-pressure-pump upper limit increasing portion.
Since the delivery pressure of the low-pressure pump is relatively low while the high-pressure pump is normal, the load acting on the low-pressure pump will not be excessively large even if the low-pressure pump is operated in the absence of a command to increase the delivery pressure of the pump device while the high-pressure pump is normal. In view of this, the pump device according to the above mode (5) is arranged such that the low-pressure pump is held operated even in the absence of such a command while the high-pressure pump is normal. This arrangement is effective to improve the operating response of the low-pressure pump when it is required to be operated in response to a command to increase the delivery pressure of the pump device. While the high-pressure pump is defective, on the other hand, the load acting on the low-pressure pump may be excessively large since the upper limit of the delivery pressure of the low-pressure pump is increased. Accordingly, the pump device according to the above mode (5) is arranged to hold the low-pressure pump off as long as a command to increase the delivery pressure of the pump device is absent while the high-pressure pump is defective. This arrangement is effective to reduce the load on the low-pressure pump, and prolong the expected life expectancy of the low-pressure pump.
(6) A pump device according to any one of the above modes (1)-(5), wherein the plurality of pumps include a low-pressure pump and a high-pressure pump having a maximum delivery pressure higher than that of the low-pressure pump, and the pump control device further comprises a low-pressure-pump drive force controlling portion for controlling a drive force of the low-pressure pump to a value corresponding to a desired value of a delivery pressure of the pump device, and high-pressure-pump on/off controlling portion for selectively turning on and of f the high-pressure pump.
The low-pressure pump is controlled such that the delivery pressure of the low-pressure pump is equal to the desired value of the delivery pressure of the pump device, so that the delivery pressure of the pump device is controlled to be the desired value. On the other hand, the high-pressure pump is selectively turned on and off, irrespective of the desired value of the delivery pressure of the pump device, so that the delivery pressure of the pump device is different from the desired value. Generally, the on/off control of a pump is easier than the control of its drive force so as to establish a particular delivery pressure. That is, the high-pressure-pump on/off controlling portion may be simpler than the low-pressure-pump drive force controlling portion. The high-pressure pump may be kept operated if it is connected to a pressure relief valve so that the fluid pressurized by the continuously operated high-pressure pump is discharged through the pressure relief valve when the delivery pressure of the high-pressure pump has been raised to a predetermined upper limit. However, this arrangement causes wasting of an electric energy due to the continuous operation of the high-pressure pump. In this respect, the on/off control of the high-pressure pump according to the above mode (6) is advantageous since the pressure relief valve is not necessary. Where the on/off control is adopted together with the pressure relief valve, the required amount of electric energy for driving the high-pressure pump is reduced by turning off the high-pressure pump before the pressure relief valve is opened.
(7) A pump device according to any one of the above modes (1)-(5), wherein the plurality of pumps include a low-pressure pump and a high-pressure pump having a maximum delivery pressure higher than that of the low-pressure pump, and the pump control device further comprises a low-pressure-pump drive force controlling portion for controlling a drive force of the low-pressure pump to a value corresponding to a desired value of a delivery pressure of the pump device, and a low-pressure-pump-failure high-pressure pump controlling portion for controlling the high-pressure pump while the low-pressure pump is defective, so that a desired value of a delivery pressure of the pump device is available based on a delivery pressure of the high-pressure pump.
Even in the event of a failure of the low-pressure pump, the desired delivery pressure of the pump device is available based on the delivery pressure of the high-pressure pump which is operated in place of the defective low-pressure pump. Where the high-pressure pump is adapted to be selectively turned on and off and is connected to a buffer provided for storing a fluid pressurized by the high-pressure pump, as described below with respect to the above (13) of this invention, the low-pressure-pump-failure high-pressure pump controlling portion may be adapted to selectively turn on and off the high-pressure pump such that the fluid pressurized by the high-pressure pump is always stored in the buffer. Where such a buffer is not provided, the low-pressure-pump-failure high-pressure pump controlling portion may be adapted to hold the high-pressure pump in an operated state. Where a high-pressure-pump drive force controlling portion is provided for controlling the drive force of the high-pressure pump to a value corresponding to the desired value of the delivery pressure of the pump device, the high-pressure-pump drive force controlling portion may be adapted to control the drive force of the high-pressure pump such that the delivery pressure of the high-pressure pump is controlled to a predetermined value which is within a range of the delivery pressure of the low-pressure pump or which is equal to the lower limit of the range of the delivery pressure of the high-pressure pump.
In the pump device according to the above mode (7) wherein the high-pressure pump is controlled while the low-pressure pump is defective so that the fluid pressurized by the high-pressure pump is always available to obtain the desired delivery pressure of the pump device, the problem caused by the failure of the low-pressure pump can be minimized.
(8) A pump device according to any one of the above modes (1)-(7), wherein the plurality of pumps include a low-pressure pump and a high-pressure pump having a maximum delivery pressure higher than that of the low-pressure pump, and the pump control device further comprises a low-pressure-pump drive force controlling portion for controlling a drive force of the low-pressure pump to a value corresponding to a desired value of a delivery pressure of the pump device and a high-pressure-pump on/off controlling portion for selectively turning on and off the high-pressure pump, and the control changing portion comprises a low-pressure-pump-failure and normal-low-pressure-pump-driver high-pressure pump controlling portion for commanding the low-pressure-pump drive force controlling portion to control the drive force of the high-pressure pump for thereby causing the high-pressure pump to perform a function of the low-pressure pump when the low-reassure pump is defective while the low-pressure-pump drive force controlling portion is normal.
In the pump device according to the above mode (8), the low-pressure-pump-failure and normal-low-pressure-pump-driver high-pressure pump controlling portion commands the low-pressure-pump drive force controlling portion to control the drive force of the high-pressure pump so that the high-pressure pump performs a function of the low-pressure pump when the low-pressure pump is defective while the low-pressure-pump drive force controlling portion is normal, that is, so that the high-pressure pump is operated in place of the low-pressure pump, to establish the desired delivery pressure of the pump device. The high-pressure-pump on/off controlling portion is not arranged to control the drive force of the high-pressure pump to a value corresponding to the desired delivery pressure value of the pump device, but is arranged to control the high-pressure pump so that the delivery pressure of the high-pressure pump is controlled to a value different from the desired delivery pressure of the pump device. When the low-pressure pump is defective, the low-pressure-pump drive force controlling portion is activated, in place of the high-pressure-pump on/off controlling portion, to control the drive force of the high-pressure pump so that the delivery pressure of the high-pressure pump is controlled to the desired delivery pressure of the pump device. Thus, the high-pressure pump performs the function of the low-pressure pump when the low-pressure pump is defective. When the low-pressure-pump drive force controlling portion controls the drive force of the high-pressure pump, this controlling portion changes suitable control parameters for controlling the high-pressure pump, depending upon the operating characteristic of the high-pressure pump, such that the control parameters used for controlling the high-pressure pump are different from those used for controlling the low-pressure pump, even if the desired delivery pressure to be established is the same. For example, the control parameters include a coefficient used to determine an electric current to be applied to electric motor motors for driving the low-pressure and high-pressure pumps, on the basis of the desired delivery pressure.
The low-pressure or high-pressure pump fails to normally operate not only when the pump per se is defective but also when the controlling portion for controlling the pump is defective as indicated above with respect to the above mode (8), or when both of the pump and the controlling portion are defective.
(9) A hydraulic pressure control system including a pump device according to any one of the above modes (1)-(5) and (8), a hydraulically operated device and a pressure control valve device interconnected between the pump device and the hydraulically operated device and operable to control a delivery pressure of the pump device such that the controlled delivery pressure is applied to the hydraulically operated device.
In the hydraulic pressure control system according to the above mode (9) which includes the pump device according to the above mode (3), the desired delivery pressure and delivery flow rate of the pump device are the delivery pressure and flow rate of the pump device which are desired for operating the hydraulically operated device. At least one of the pumps of the pump device which should be operated is selected by the pump selecting portion of the pump device, on the basis of at least one of the desired delivery pressure and flow rates of the pump device, and on the basis of the information defining the operating ranges of the pumps.
(10) A hydraulic pressure control system according to the above mode (9), wherein the plurality of pumps include a low-pressure pump and a high-pressure pump having a maximum delivery pressure higher than that of the low-pressure pump, and the pump control device comprises (a) a pump selecting portion operable when the plurality of pumps are normal, for selecting at least one of the plurality of pumps which should be operated, on the basis of at least one of a desired delivery pressure and a desired delivery flow rate of the pump device, and on the basis of information defining an operating range in which each of the plurality of pumps should be operated, which operating range is determined by a capacity of the each pump, and (b) a low-pressure-pump drive force controlling portion for controlling a drive force of the low-pressure pump to a value corresponding to a desired value of the delivery pressure of the pump device, and wherein the control changing portion comprises a low-pressure-pump upper limit increasing portion for increasing an upper limit of a delivery pressure of the low-pressure pump to thereby change the operating range of the low-pressure pump when the high-pressure pump becomes defective, such that the upper limit is higher while the high-pressure pump is defective than while the high-pressure pump is not defective, the pump control device further comprising a high-pressure-pump failure pressure control valve controlling portion operable when the high-pressure pump becomes defective, for commanding the pressure control valve device to operate, in place of the low-pressure-pump drive force controlling portion, to control the delivery pressure of the low-pressure pump to be applied to the hydraulically operated device, at least when the desired value of the delivery pressure of the pup device is in a range between two values of the upper limit of the delivery pressure of the low-pressure pump before and after the upper limit has been increased by the low-pressure-pump upper limit increasing portion.
Before the upper limit of the delivery pressure of the low-pressure pump has been increased by the low-pressure-pump upper limit increasing portion, the drive force of the low-pressure pump can be controlled such that the pressure to be applied to the hydraulically operated device is controlled to the desired delivery pressure of the low-pressure pump, and therefore the pressure control valve device need not be controlled. After the upper limit of the delivery pressure of the low-pressure pump has been increased, however, the delivery pressure of the low-pressure pump cannot be accurately controlled by controlling the drive force of the low-pressure pump. Accordingly, after the upper limit of the delivery pressure of the low-pressure pump has been increased, the pressure control valve device rather than the low-pressure pump is controlled so that the pressure to be applied to the hydraulically operated device is controlled with higher accuracy. It is noted that after the upper limit of the delivery pressure of the low-pressure pump has been increased, the high-pressure-pump-failure pressure control valve controlling portion may command the pressure control valve to operate, in place of the low-pressure-pump drive force controlling portion, to control the delivery pressure of the low-pressure pump, even when the desired value of the delivery pressure of the low-pressure pump is lower than the normal upper limit (upper limit which has not been increased).
The pump control device may comprise, in addition to the pump selecting portion and the low-pressure-pump drive force controlling portion, a high-pressure-pump on/off controlling portion for selectively turning on and off the high-pressure pump, or a high-pressure-pump drive force controlling portion for controlling the drive force of the high-pressure pump to a value corresponding to a desired value of the delivery pressure of the pump device.
(11) A hydraulic pressure control system including a pump device according to the above mode (7), a hydraulically operated device and a pressure control valve device interconnected between the pump and device and the hydraulically operated device and operable to control a delivery pressure of said pump device such that the controlled delivery pressure is applied to the hydraulically operated device.
(12) A hydraulic pressure control system according to the above mode (11), wherein the pump control device further comprises a low-pressure-pump-failure pressure control valve controlling portion operable at least when the low-pressure-pump-failure high-pressure-pump controlling portion is in operation, for commanding the pressure control valve device to control the delivery pressure of the high-pressure pump to be applied to the hydraulically operated device.
For instance, the pump control device may include a high-pressure pump on/off controlling portion or a high-pressure-pump drive force controlling portion, for controlling the high-pressure pump. The high-pressure pump on/off controlling portion is adapted to selectively turn on and off the high-pressure pump. On the other hand, the high-pressure-pump drive force is adapted to control portion for controlling the drive force of the high-pressure pump to a value corresponding to the desired value of the delivery pressure of the pump device. In the case where the high-pressure pump on/off controlling portion is provided to control the high-pressure pump, the drive force of the high-pressure pump is not controlled depending upon the desired delivery pressure, but the high-pressure pump is selectively turned on and off such that the delivery pressure is held at a suitable value not determined by the desired value, while the delivery pressure of the high-pressure pump is controlled to the desired value by the pressure control valve device at least when the low-pressure pump is defective, so that the pressure to be applied to the hydraulically operated device is controlled to the desired value of the delivery pressure as obtained by the low-pressure-pump drive force controlling portion. In the latter case where the high-pressure-pump drive force controlling portion is provided to control the delivery pressure of the high-pressure pump, the accuracy of control of the delivery pressure of the high-pressure pump by the high-pressure-pump drive force controlling portion is comparatively low when the desired delivery pressure is lower than the normal upper limit of the low-pressure pump. In this case, therefore, the delivery pressure of the high-pressure pump is controlled by the pressure control valve device so that the delivery pressure is controlled to the desired value with high accuracy. The low-pressure-pump-failure pressure control valve controlling portion may be adapted such that the pressure which is normally applied to the hydraulically operated device from the low-pressure pump is applied from the pressure control valve device. Alternatively, the low-pressure-pump-failure pressure control valve controlling portion may be adapted such that the desired pressure is always applied from the pressure control valve device to the hydraulically operated device.
(13) A hydraulic pressure control system according to any one of the above modes (9)-(12), where the plurality of pumps include a low-pressure pump and a high-pressure pump having a maximum delivery pressure higher than that of the low-pressure pump, and the pump device includes a buffer connected to the high-pressure pump for accommodating a working fluid pressurized by the high-pressure pump.
The buffer used in the hydraulic pressure control system according to the above mode (13) has a volume which is smaller than an accumulator used in an ordinary hydraulic pressure control system and larger than a damper usually provided for reducing a delivery pressure pulsation of a pump. Usually, an accumulator has a volume enough to store an amount of pressurized working fluid sufficient to activate a hydraulically operated device a plurality of times, while a damper has a volume smaller than an amount of the working fluid delivered by one pumping action of the pump. The volume of the buffer in question is smaller than the amount of the working fluid required for performing a single operation of the hydraulically operated device, and is larger than the amount of the fluid delivered by one pumping action of the high-pressure pump.
Where the buffer is provided, the number of on/off operations of the high-pressure pump can be reduced. The pressurized fluid stored in the buffer permits the hydraulically operated device to be controlled as needed, without an operation of the high-pressure pump. Accordingly, the provision of the buffer results in an increase in the service life of the high-pressure pump.
The feature of the above form (13) of the invention is available independently of any one of the features of the above modes (1)-(12).
(14) A hydraulic pressure control system according to the above mode (13), wherein the high-pressure pump is of an internal fluid leakage type which permits the working fluid remaining in the buffer to be discharged due to an internal leakage of the working fluid through the internal fluid leakage type high-pressure pump after an operation of the hydraulically operated device.
The pressurized fluid which would remain in the buffer even after the operation of the hydraulically operated device may cause deterioration of the buffer and the related hydraulic piping due to the continued application of the pressure of the pressurized fluid in the buffer. In the hydraulic pressure control system according to the above mode (14) wherein the high-pressure pump is of an internal fluid leakage type, the pressurized fluid remaining in the buffer may be discharged through the high-pressure pump, so that the expected service life of the buffer may be prolonged. In addition, the hydraulic pressure control system does not require any special means for discharging the pressurized fluid from the buffer, and the system can be made relatively simple in construction.
(15) A hydraulic pressure control system according to the above mode (13) or (14), wherein the pump control device further comprises a buffer pressure controlling portion for turning off the high-pressure pump before the buffer is filled with the working fluid pressurized by the high-pressure pump, and turning on the high-pressure pump before the buffer has become empty.
In the hydraulic pressure control system according to the above mode (15), the buffer pressure controlling portion prevents filling of the buffer with the pressurized fluid, which would cause overloading of the high-pressure pump and its drive device, or cause opening of a pressure relief valve if disposed in parallel connection with the buffer. The buffer pressure controlling portion further prevents the buffer from becoming empty. If the buffer becomes empty, the rate of flow of the fluid to the hydraulically operated device is limited to the delivery flow rate of the high-pressure pump.
(16) A hydraulic pressure control system according to the above mode (15), wherein the buffer pressure controlling portion comprises high-pressure-pump turning-off portion for turning off the high-pressure pump when the pressure of the working fluid stored in the buffer as a result of an operation of the high-pressure pump has been increased to a predetermined upper threshold which is lower than a highest pressure of the working fluid that can be stored in the buffer.
(17) A hydraulic pressure control system according to the above mode (16), wherein the high-pressure-pump turning-off portion comprises a buffer upper-threshold-pressure detecting device for detecting that the pressure of the working fluid stored in the buffer has been increased to the predetermined upper threshold.
For instance, the buffer upper-threshold-pressure detecting device includes a pressure detecting device for detecting the pressure of the working fluid stored in the buffer. Where the buffer includes a housing, a piston movably received in the housing, and biasing means for biasing the piston, the buffer upper-threshold-pressure detecting device may include a piston position detecting device for detecting that the piston has been moved to a position at which the pressure of the fluid in the buffer is equal to the predetermined upper threshold.
(18) A hydraulic pressure control system according to any one of the above modes (15)-(17), wherein the buffer pressure controlling portion comprises a high-pressure-pump turning-on portion for turning on the high-pressure pump when the pressure of the working fluid stored in the buffer after the high-pressure pump is turned off has been reduced to a predetermined lower threshold which is higher than a lower limit pressure of the working fluid that can be stored in the buffer.
(19) A hydraulic pressure control system according to the above mode (18), wherein the high-pressure pump turning-on portion comprises a buffer lower-threshold-pressure detecting device for detecting that the pressure of the working fluid stored in the buffer has been reduced to the predetermined lower threshold.
(20) A hydraulic pressure control system according to any one of the above modes (13)-(19), wherein the pump device further includes a pressure relief valve connected to the high-pressure pump in parallel with the hydraulically operated device.
In the hydraulic pressure control system according to the above mode (20), the pressure relief valve is opened when the delivery flow rate of the high-pressure pump becomes higher than the rate of flow of the pressurized fluid to the hydraulically operated device where the buffer indicated above is not provided or where the buffer is filled with the pressurized fluid. Accordingly, the high-pressure pump and its drive device is prevented from being overloaded.
(21) A hydraulic pressure control system according to the above mode (20), wherein said pump control device includes a high-pressure-pump turning-off portion operable when said pressure relief valve is opened, for turning off said high-pressure pump.
The pressure relief valve is opened and the high-pressure pump is turned off when the delivery flow rate of the high-pressure pump becomes higher than the rate of low of the pressurized fluid to the hydraulically operated device where the buffer is not provided. Where the buffer is provided, the predetermined highest pressure of the fluid below which the fluid is stored in the buffer is usually lower than the relief pressure at which the pressure relief valve is opened. The high-pressure pump operated to store the pressurized fluid in the buffer is turned off when the pressure of the fluid in the buffer has been increased to the predetermined highest value. The high-pressure pump cannot be turned off in some cases in the event of a failure of the pressure sensor or the pump control device. In this cases, the pressure relief valve is opened. The opening of the pressure relief valve can be detected by detecting a movement of a valve member of the pressure relief valve, using a photoelectric sensor, or by detecting a sound of the working fluid flowing through the opened pressure relief valve, by using a microphone.
(22) A hydraulic pressure control system according to any one of the above modes (9)-(21), wherein said hydraulically operated device includes a hydraulically operated brake having a wheel brake cylinder which is hydraulically activated to brake a wheel of a vehicle.
In the hydraulic pressure control system according to the above mode (22), the desired value of the delivery pressure of the pump device is the desired pressure of the working fluid in the wheel brake cylinder. Where the pump device includes a low-pressure pump device and a high-pressure pump device, the low-pressure pump is operated when the desired value of the pressure in the wheel brake cylinder is relatively low, and the high-pressure pump is operated when the desired value of the wheel brake cylinder pressure is relatively high.
(23) A hydraulic pressure control system according to the above mode (22), wherein said buffer has a volume not larger than 10 cc.
The upper limit of the volume of the buffer indicated above is smaller than an amount of the working fluid required for activating four wheel brake cylinders for a braking operation with the nominal largest braking force of the hydraulic pressure control system in the form of a hydraulic barking system. The volume of the buffer may be 5 cc or smaller, or 3 cc or smaller.